This invention relates to a composition of reactive toluene diisocyanate bottoms and a method of producing them. More particularly, this invention relates to reduction of hydrolyzable chloride in crude toluene diisocyanate.
The term "hydrolyzable chloride" refers to labile chlorine atoms which are free, ionically or covalently bonded within a compound, but have more ionic character than, for example, the chlorine atom present in chlorobenzene. Hydrolyzable chloride concentration may also be referred to as chloride equivalent or chloride level.
Toluene diisocyanates (hereinafter TDI) are commonly prepared by phosgenation of toluene diamines. Typical processes for the phosgenation of amines may be found in U.S. Pat. Nos. 2,680,127: 2,822,373: and 3,781,320. After the phosgenation of toluene diamines to form TDI, the product diisocyanate is generally distilled from the reaction mixture in which it is prepared. This reaction product mixture, after phosgenation but prior to distillation of TDI monomer, shall hereafter be referred to as crude TDI. At the conclusion of the distillation, the residue of the distillation is commonly referred to as distillation bottoms or, more simply, just bottoms. The bottoms normally contain a quantity of high boiling residue composed of materials such as alpha and omega-isocyanato-biurets, polycarbodiimides, diisocyanatocarbodiimides, polyuretidinediones, isocyanurates and various other isocyanate adducts, including hydrolyzable chloride containing compounds. This residue is typically discarded.
The disposal of this residue is a serious problem to the TDI industry. It is costly and poses safety problems. The handling of this material is a problem because special equipment is needed to move and store it prior to disposal.
A number of processes for reduction of hydrolyzable chloride have been developed, primarily for methylene diphenyl diisocyanate. Some of the processes involve reduction of acidity levels. U.S. Pat. No. 3,179,680 (Inventor: Kober: Assignee: Mathieson Chemical Corp ) discloses that the concentration of hydrolyzable chloride in organic isocyanates may be reduced by heating the organic isocyanates in the presence of small amounts of water. However, water is reactive with isocyanates and may reduce yields or cause other processing problems. U.S. Pat. No. 3,219,678 (Inventor: Kober: Assignee: Mathieson Chemical Corp.) discloses that reduction of hydrolyzable chloride can be achieved by heating an isocyanate containing hydrolyzable chloride at temperatures considerably above those required and used for the cleavage of carbamoyl chlorides to form organic isocyanates and hydrogen chloride (hereinafter HCl), and then, passing an inert gas through the isocyanate to remove the HCl. This process, however, fails to address the problem of reformation of hydrolyzable chloride containing compounds. U.S. Pat. No. 3,857,871 (Inventor: Hatfield et al.: Assignee: The Upjohn Co.) discloses that acidity and hydrolyzable chloride levels of polymethylene polyphenyl polyisocyanate (hereinafter PMDI) are reduced and reactivity increased by exposing the polyisocyanate in a liquid state at 350.degree. F. to 450.degree. F. to countercurrent treatment with an inert gas. However, use of heat and an inert gas alone does not appear to maximize removal of hydrolyzable chloride to produce desirably reactive TDI bottoms. Thus, it would be desirable to remove sufficient hydrolyzable chloride from crude TDI to produce both reduced hydrolyzable chloride TDI monomer and commercially useful distillation bottoms.